(Reuters) – U.S. regulators have approved AstraZeneca Plc’s Imfinzi treatment for use against an aggressive type of lung cancer in previously untreated patients, the group said on Monday, as it moves to expand its specialization in tackling the disease.

The treatment, when combined with chemotherapy, was approved by the U.S. Food and Drug Administration for adult patients with extensive-stage small cell lung cancer, the British drugmaker said.

AstraZeneca has already carved out a niche for Imfinzi, which enables the immune system to detect and attack certain cancer cells, in the more common non-small cell lung cancer (NSCLC) setting, with approval for tumors that have only spread in the chest.

Merck & Co’s immunotherapy bestseller Keytruda has become the standard treatment option for the larger group of NSCLC patients that are diagnosed at a more advanced stage of the disease.

In the small-cell type of cancer, Astra is seeking to catch up with Roche, whose immunotherapy Tecentriq won U.S. approval for the disease type a year ago.

Small cell lung cancer is a highly aggressive, fast-growing form of cancer that typically recurs and progresses rapidly despite initial response to chemotherapy.

Lung cancer is the leading cause of cancer deaths among both men and women, accounting for about one fifth of all deaths from the disease.

(Reporting by Ludwig Burger in Frankfurt and Aakash Jagadeesh Babu in Bengaluru; Editing by Shounak Dasgupta and Jan Harvey)